In the art of manufacturing semiconductor devices and particularly integrated circuits, introduction of dopant elements into semiconductor bodies is an essential step. Numerous methods of placing atoms of dopant elements, such as boron, arsenic and phosphorus, in silicon and other semiconductors are known and practiced in the art.
In a long established doping process, a source, such as a crucible containing the dopant element, and a boat carrying wafers of the semiconductor body to be doped, are placed in a furnace. An appropriate atmosphere is maintained in the furnace. The dopant source is then raised to a sufficient temperature to vaporize dopant atoms and transport them to the surface of the semiconductor wafers. The wafers are held at a temperature sufficient to diffuse into them the dopant atoms that contact the wafer surfaces.
In a more modern version of the process just described, the dopant atoms are contained on the surfaces of a porous or non-porous solid dopant source, usually in a planar form. A planar source is disposed in a furnace, usually between and closely spaced from two semiconductor bodies to be doped. The assembly of dopant sources and wafers is placed in a furnace in a controlled atmosphere and the temperature is raised so that dopant atoms are evolved from the source, contact the semiconductor body and are diffused into it. Examples of such porous sources are referred to in U.S. Pat. Nos. 4,525,429 for "Porous Semiconductor Dopant Carriers" and 4,596,716 for "Porous Silicon Nitride Semiconductor Dopant Carriers." An example of a non-porous source is described in U.S. Pat. No. 3,920,882 for "N-Type Dopant Source". According to the latter patent, in its Part D of Example 3, an embodiment of an antimony dopant source evolved 0.03 grams per minute of material during a diffusion at 1220.degree. C. This rate of weight loss means that such dopant sources will not have a long life, i.e. will not survive a large number of doping cycles.
Liquid dopant sources may also be painted on a semiconductor body to be doped. Typically the body is spun while a liquid is dropped on it. The spinning spreads the liquid dopant source uniformly and dries it. Thereafter, the semiconductor body is heated to diffuse dopant atoms from the dried layer into the semiconductor body. This process is particularly undesirable because of the direct contact of the semiconductor surface with a foreign object, resulting in surface contamination and damage. Examples of such dopant sources are described in U.S. Pat. Nos. 4,571,366 for "Process for Forming a Doped Oxide Film and Doped Semiconductor" and 4,490,192 for "Stable Suspensions of Boron, Phosphorus, Antimony and Arsenic Dopants."
In still another related doping process, mixtures, such as solutions, pastes or slurries, containing dopant atoms, usually as an oxide, along with other ingredients are painted on the surface of a non-reacting substrate, such as silicon. The mixture is dried, usually to drive off a solvent, to form a solid or semi-solid layer on the substrate. These sources are placed in a furnace with semiconductor bodies to be doped. Thereafter, the temperature is raised to transport dopant atoms to the semiconductor body for diffusion into it. An example of such a dopant source is described in U.S. Pat. No. 4,588,455 for "Planar Diffusion Source." In our experiments, embodiments of sources of the type described in that patent and incorporating antimony produced highly unpredictable doping and dopant concentrations in semiconductor bodies.
A more recently developed doping technique that is widely used in integrated circuit manufacture is ion implantation. In that process a gaseous source of dopant atoms is introduced to a chamber where the dopant atoms are ionized. The ionized atoms are then accelerated toward and become embedded in the semiconductor wafer being doped. This technique provides good control of the dosage and location of doping. However, crystal structure of the doped semiconductor is damaged by the high energy dopant ions, particularly when the dosage is high. Moreover, the apparatus required for ion implanation is extremely complex and expensive. In addition, the dopant source gases used are highly toxic.
A low cost, long lasting, inexpensive semiconductor dopant source is needed that can predictably dope semiconductor wafers, particularly silicon. Of the dopants that produce n-type conductivity, antimony is a particularly useful one for widely used processes. For example, semiconductor manufacturers frequently wish to establish "buried" layers in their devices. Buried layers are relatively deeply disposed in a silicon wafer and so are prepared in one of the earlier steps of a process that may include many subsequent, relatively high-temperature processes. The dopant atoms in the buried layer, e.g. a buried collector of a bipolar transistor or an isolating buried layer in an MOS device, diffuse in the subsequent high-temperature processes, spreading out junctions and moving their locations. Antimony is a desirable dopant for buried layers since it diffuses less than other n-type dopants (phosphorus and arsenic). Moreover, antimony is less volatile than arsenic in the conditions of temperature and atmosphere that are used to grow silicon epitaxially in the course of manufacturing integrated circuits. As a result, there is less autodoping of a silicon epitaxial layer by an antimony doped substrate than by an arsenic doped substrate. Therefore, a low cost, long lasting, inexpensive doping source that can predictably dope silicon with antimony would be particularly useful.